The present invention relates to an air servo valve constituted so as to perform spool switching using an electromagnetic actuator of a magnet movable type as a drive source.
Conventionally, as an equipment for controlling fluid pressure supplied to a fluid pressure cylinder or the like, a switching valve of a spool type has been known. The switching valve of a spool type is constituted such that an electromagnet is used as a drive source for driving a spool and the spool is driven by a movable iron core of the electromagnet. However, such an electromagnet is suitable for a case that the spool is switched to two positions but it is unfit for a case that flow rate control is performed by performing feedback control on the operation position of a spool.
In view of the above, there is an example that, when flow rate control is performed by performing feedback control on the operation position of the spool, an electromagnetic actuator of a magnet movable type is used as a drive source, for example, such as described in JP-A 06-38486 publication. In this electromagnetic actuator, a rod-like permanent magnet is disposed in the interior of a coil wound in a cylindrical shape as a movable body, and the permanent magnet is caused to displace by interaction between flux generated by current flow to the coil and flux due to the permanent magnet so that spool switching is performed. Then, the operation position of the spool is detected by a displacement sensor and the electromagnetic actuator is feedback-controlled on the basis of a detection signal of the sensor so that flow rate control is performed.
However, since the above-described known electromagnetic actuator uses a rod-like permanent magnet as a movable body, there is a problem that a ratio of generated thrust/permanent magnet weight becomes small due to the size of the magnet and response is poor. Also, since N pole and S pole are magnetized in an axial direction of the permanent magnet, there is also a problem that the actuator is easily influenced by lateral load at a time of actuation thereof. Further, in case that flow rate control is performed by conducting feedback control on the operation position of the spool, when such a constitution is employed that the displacement of the permanent magnet is detected by a magnetic sensor, it is convenient because it is unnecessary to detect the displacement of the spool directly. However, since the flux density is not changed linearly, it is much difficult to detect the displacement of the permanent magnet accurately.
A technical object of the present invention is to solve the above-described conventional defect in an air serve valve using an electromagnetic actuator of a magnet movable type as a drive source for spool switching when a rod-like permanent magnet where N pole and S pole are magnetized in an axial direction is used as a movable body, and to constitute a air servo valve which is excellent in response and excellent in operation stability and which allows the displacement of a spool to be accurately detected via the movable body to enable flow rate control with a high precision.
In order to solve the above problem, an air servo valve according to the present invention comprises: a valve switching portion having a plurality of ports, a valve hole communicating with the ports, and a spool which is slidably inserted in the valve hole to switch flow paths among the ports; a driving portion having a cylindrical yoke provided with a pair of pole teeth opposed to each other, at least one exciting coil wound on the yoke, a cylindrical permanent magnet which is disposed in a hollow portion of the yoke movably in an axial direction of the yoke and where N pole and S pole are magnetized in a radial direction, and a magnet holder made of non-magnetic body, which couples the permanent magnet and the spool to each other; a displacement sensor which is disposed so as to position on a center axial line of the magnet inside the permanent magnet and detects the displacement of the permanent magnet from change of flux density; and a control portion which compares a detection signal from the displacement sensor with a set value to adjust a degree of valve opening by controlling the driving section on the basis of the deviation of the detection signal from the set value.
In the air servo valve having the above-described constitution, when the exciting coil is energized, one pole tooth of the yoke becomes N pole and the other pole tooth becomes S pole in response to the direction of the current. Then, when the magnetic pole generated in these pole teeth and the magnetic pole on the side of an outer periphery of the permanent magnet are different poles, an attracting force acts between these magnetic poles, but when they are the same pole, a repelling force acts therebetween. Therefore, the permanent magnet displaces in the axial direction due to these acting forces so that spool switching is performed. At this time, since the cylindrical permanent magnet is used as the movable body, the thrust relative to the weight of the movable body can be made larger as compared with the conventional article using the rod-like permanent magnet and the air servo valve of the present invention is superior to the conventional article in response.
On the other hand, the displacement of the permanent magnet is detected as the displacement of the spool by the displacement sensor, and the detection signal is fed back to the control circuit. Then, in this control circuit, the detection signal is compared with the set value, a control signal is outputted from the control circuit to the driving portion such that the deviation becomes zero, and the amount of displacement of the permanent magnet, namely, the amount of displacement of the spool is adjusted so that flow rate control is performed. At this time, since the flux density of the cylindrical permanent magnet which has been magnetized in the radial direction varies linearly on the center axial line, the displacement sensor is disposed so as to position on the center axial line within the hollow portion of the permanent magnet, so that the displacement of the permanent magnet can be detected accurately and flow rate control can be made with a high precision.
According to a preferred specific constitution aspect, the yoke comprises a cylindrical outer cylinder portion and a pair of inner cylinder portions which are concentric to the outer cylinder portion and are opposed to each other with a constant gap, the inner cylinder portions form the first pole tooth and the second pole tooth, and the exciting coil is accommodated in a space portion between the inner cylinder portions and the outer cylinder portion.
According to another specific constituent aspect of the present invention, the magnet holder is formed in a cylindrical shape and is accommodated in the hollow portion surrounded by the first pole tooth and the second tooth of the yoke so as to be capable of displacing in an axial direction, and the permanent magnet is fitted on an outer peripheral face of the magnet holder.
Also, in the present invention, a sensor holder extends from a cover covering an end portion of the yoke towards the inside of the permanent magnet in a center axial line of the permanent magnet, and the displacement sensor is held by the sensor holder.
Further, according to the present invention, such a constitution can be employed that a plurality of exciting coils wound in directions inverse to each other are arranged in the axial direction of he yoke and the magnitudes of currents flowing in respective exciting coils can be controlled individually.
With such a constitution, since the magnetic pole generated the first pole tooth and the second pole tooth of the yoke and the intensity thereof are determined according to the sum of magnetic fields generated by two exciting coils, the moving direction of the permanent magnet and the spool can be switched in forward and backward directions by only controlling the magnitudes of the currents individually without switching directions of currents flowing in two exciting coils forward and backward.
In the present invention, such a constitution is preferably employed that a current sensor for detecting current flowing in the exciting coil is provided and the coil current detected by the current sensor is fed back to the control portion so that a response delay of the coil current to the coil voltage is compensated for.
Also, in the present invention, a manual operating mechanism for switching the spool manually can be provided. The manual operating mechanism comprises a handle which is attached to a valve casing so as to be rotationally operable about an axial line of the handle itself, a pin which is provided at a distal end of the handle in an eccentric state, and an engaging groove which is formed the spool or the magnet holder to be engaged with the pin.
In the present invention, such a constitution is preferably employed that the spool is maintained in a neutral position at a time of non-excitation to the exciting coil. As means therefor, there is a method where the yoke is constituted so as not to be saturated magnetically by a magnetomotive force of the permanent magnet so that the magnetic saturation characteristic of the yoke is utilized, a method where return springs are provided on both ends of the spool, or the like.